The invention relates to an apparatus to reduce the sagging of essentially planar outside walls of vacuum tanks, especially on vacuum tape coating installations, and to prevent the resultant displacement of components fastened to these outside walls, such as the rollers of a winding system.
In vacuum tanks there is generally a danger of the deformation and especially of the sagging in of the tank walls due to the great difference in the pressure on the two sides of the outside walls. This is true of all tanks of all sizes and geometric shapes. In a great number of vacuum tanks, deformation of the kind described above is of minor importance and can be tolerated.
However, there are applications, such as the vacuum coating of tapes, in which the sagging in of the tanks is undesirable or unacceptable. Tape coating apparatus include, as a rule, a stationary vacuum tank with a movable outer wall--the so-called receptacle plate. On this plate there is fastened a winding system for carrying the tape that is to be coated. This winding system includes a plurality of rollers disposed parallel to one another and journaled between the tank plate and a back plate. On account of the extremely slight thickness of the tape or film to be coated and the high speed with which the film runs in the apparatus, the requirements as to the parallelism of the rollers are very great. Even a slight misalignment renders the film unusable.
During the coating operation, i.e., when a vacuum prevails in the tank, the atmospheric pressure against the outer side of the receptacle plate causes the tank plate to sag inward. Since the back plate is largely free of external forces, the rollers get out of parallel to a degree depending on their shape and on how they are mounted.
These undesirable departures from the parallelism of the rollers can be countered by a more complex construction of the rollers. For example, the axles of the rollers are made to run all the way through, which creates the possibility of providing a loose bearing seat at the receptacle plate end to isolate it largely from the movement of the wall. A disadvantage in this case is the greater use of material as well as the low natural frequency of flexural vibrations of the long axle.
Another possibility for reducing o preventing the receptacle plate from sagging is to provide flexurally stiff structural members on the outside of the plate as reinforcement. This, however, has considerable disadvantages as regards production, so that the manufacture of such receptacle plates is more complex and thus more expensive than that of plain plates.
The present invention, then, is addressed to the problem of creating an apparatus which will reduce the sagging in of essentially planar outside walls on vacuum tanks, prevent resultant displacements of rollers, and will be simpler and less expensive to make than apparatus known heretofore.
This problem is solved according to the invention by placing, for example, a spacer rod between the outer walls of the vacuum chamber, if possible in the center of the receptacle plate. It will be set at a minimal distance from the back wall.
Upon the evacuation of the vacuum tank, the forces on the back wall and the receptacle plate will counteract one another through this prop and thus advantageously reduce the sagging.
In a mathematical model, the result was a reduction of the maximum sag from 4 mm, for example, to 0.4 mm, which advantageously results in a 90% reduction of the sagging of the receptacle plate.
Problems with the sagging of outer walls of vacuum tanks of all kinds can basically be reduced in this manner.